Disposable net assemblies for apparatus for collecting floating debris

ABSTRACT

A disposable net assembly is provided for a trap for collecting floatable debris in a waterway or combined sewer system. The net assembly includes a knitted bag-shaped mesh net having a frame surrounding the mouth of the net with the net secured around its rim to the frame. The net may be formed of an inner layer and an outer layer of mesh with the openings of the inner layer being substantially larger than the openings of the outer layer. The frame may be formed of a plastic molded material having side members tapered in the vertical direction to facilitate the changing of the netting assemblies and tapered in the downstream direction to lock into place under the force of the flow. Several embodiments of the frame members have projections thereon which cooperate with a clamping element to hold the net to the frame. Some embodiments of the members have parts that lock together with a post and hole construction while others employ a tension band to clamp the net to the projections on the frame. The net is preferably secured around its rim to the frame, with the mouth of the net extending around the outside and upstream side of the frame and over the surface having the projections. The net is preferably formed of a high strength and high stretch yarn, with rolled sewn seams and having reinforcing on the seams and on high stress areas of the net.

The present invention generally relates to the collection and removal oftrash or floating debris from waterways and, more particularly, tosystems designed for use in combined sewer systems or storm drainconduits to trap water borne trash for removal.

BACKGROUND OF THE INVENTION

Trash and debris floating on the surfaces of waterways or alongshorelines and beaches is a highly visible form of water pollution,which is receiving attention for its adverse, polluting effect and forits unaesthetic appearance on the surfaces of lakes and other waterbodies. One type of system for the collecting and removing of floatingdebris has consisted of arrays of disposable mesh nets installed inreceiving bodies of water in the flow path of a sewer outlet,particularly in applications referred to as “Combined Sewer Overflows”or “CSOs”. Such systems are described in Vol. 2, No. 3, of Fresh CreekTechnologies, Inc. “Shorelines” newsletter. Systems of this type areeffective in collecting floatables or trash for removal and are shown inFresh Creek Technologies, Inc. Netting Trashtrap™ Product Bulletin.Improvements in such devices are described in U.S. Pat. No. 5,562,819,owned by the assignee of the present application, which provides anunderground, in-line apparatus for trapping and collecting debris in asewer or storm flow conduit, a secondary trap which provides continuedprotection when primary collection traps are full, a system whichsignals when primary bags or nets are full and servicing is required,and a trapping facility in which bags or nets may be replaced withoutloss of trapping protection during servicing.

More specifically, the device in the patent referred to above includesan enclosure or chamber with an inlet and an outlet each adapted to beconnected to a sewer, storm drain conduit or outflow. A debris removingsystem is disposed within the chamber between the inlet and the outletfor trapping and collecting water borne debris entering at the inlet andthereby providing for an outflow of substantially debris-free water. Theenclosure includes an access opening comprising upper doors or hatchesor access hatches in the enclosure sized to allow the debris removingsystem to be removed and replaced. The debris removing systemspecifically includes a perforated container having an open end facingthe inlet of the chamber. The perforated container includes a nettingassembly that traps and collects the trash or floating debris. Thecontainer is in the form of a netting assembly having a flexiblebag-shaped mesh net attached to a frame. The netting assembly isattached to lifting structure having supports or handles for allowingthe frame and net to be lifted out when the net is full of captureddebris. In some applications, a bypass weir or screen is provided tonormally direct flow from the chamber inlet through the open end of thenet while allowing flow to bypass the net and flow to the chamber outletwhen the net is full of debris.

Sensing and signaling elements are typically provided for sensing andsignaling the passage of solid debris around the net when the net isfull of debris and is in need of servicing. The sensing and signalingelements may include mechanical structure which permits passage ofwater, but is displaced by impingement of solid debris flowing aroundthe nets. Displacement of such mechanical structure signals when the netis full of debris, for example, by actuating a visible flag above groundor by actuating an electrical switch which activates an abovegroundindicator or remote indicator. The sensing and signaling may include anoptical sensor for detecting the passage of debris around the nettingassembly. Upon detection of debris, the optical sensor emits a signalindicating that the trap is full of debris. The signal may also activatean aboveground indicator or a remote indicator.

Multiple trap systems are employed in which the enclosure includesside-by-side trap assemblies. Such systems may be configured such that,upon filling of the first trap, the flow and debris can be diverted overa bypass weir disposed between the inlet ends of the first and secondtraps so that flow is thereby directed through the second trap andoverflow debris is trapped and collected. Closure panels may be providedin a stationary frame structure disposed adjacent the inlet ends of thetraps in either the single-trap systems or the multitrap systems torestrain debris from flowing through the chamber during servicing.

The reliability of debris removing systems depends on the strength ofthe mesh nets and on the manner in which the net material is fabricatedinto the disposable net assemblies. The resultant hydraulic forces are afunction of the velocity of the flow of water through the mesh of thenets as well as on the pressure exerted on the debris trapped by thenets. There are many outfalls where extreme forces exist that are toohigh for standard and commonly available materials or for materials madeby normal fabrication practices to last.

Furthermore, the operation of such debris removal systems results in thenets filling with floatable materials over time as one or more overflowsoccur. In the process, large objects such as plastic bottles and sheetsof plastic wrapping materials tend to cover and blind openings of themesh, which reduces the overall effective area of the filter and resultsin higher hydraulic loading on the mesh, contributing to a higherpressure drop through the system and increased loads, and excessiveforces on the nets.

Accordingly, a need exists for stronger and more reliable mesh nets inthe traps of floatable debris collecting systems, and particularly fornet assemblies that can be easily constructed and easily replaced.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide a strongerand more reliable mesh net for the traps of systems for collectingfloatable debris than have been provided by the prior art. A furtherobjective of the invention is to provide a reliable net assembly forsuch systems that can be easily constructed and easily replaced.

According to principles of the present invention, disposable mesh netsare provided for debris traps that can withstand higher level of forcesthan can nets of the prior art. Such nets are, according to a preferredembodiment, made with a high strength and high stretch yarn and may beprovided with reinforcing tape on seams and high stress areas of the netmaterial. The flexible, stretchable mesh material allows for an increasein the free area of the mesh as the nets expand under hydraulic loads asthe nets fill. High elasticity materials are those that are elasticenough, either due to their composition or the ways in which they areknitted, to allow the nets to deform when clogged with debris andthereby expand to allow flow paths around the trapped debris to minimizepressure. Nylon that has these properties would, for example, besuitable. The knit of the mesh material yarn is selected to produce thedesired aperture size and maximize the breaking strength of the finishedmaterial and ability to maintain constant aperture. The material used inthe manufacturing process enables the flexible mesh to maintain aconsistent percentage of free area as the nets fill and expand. Thematerial is fabricated into the form of a bag-shaped mesh net from flatmaterial with seams that are rolled and stitched to give a strengthgreater than the knitted material itself.

Further according to principles of the invention, a netting assembly isprovided with structure for holding the mouth of the bag-shaped net inan open position and which can be easily and securely attached to thenetting material. In the preferred embodiment, the structure includes aone-piece frame that is provided with a strap configured to hold thenetting material in place on the frame. The strap fits in a recessedgroove molded into the outer perimeter of a generally rectangular moldedplastic frame. Rows of raised buttons integrally molded into the frameextend from the bottom of the groove such that the mesh net will besandwiched between the strap and the buttons. The frame is sized toprovide sufficient strength to counter the hydraulic forces on the net.This particular embodiment of the invention is particularly suited toresist hydraulic forces in the dirty environment wherein the nettingassemblies trap floating debris from waterways, sewers or storm drainconduits, as the frame assembly requires no removable locks, pins,clamps, brackets or other devices to hold down the netting material tothe frame. The structure has a minimum of parts to collect debris whilepermitting the netting assembly to be loosened from the system with apair of gloved hands.

In other embodiments, the netting assemblies are provided with a twopart molded plastic rectangular frame, the parts of which clamp togetherwith the knitted mesh material around the mouth of the net clampedtherebetween, thereby evenly distributing the forces around the mouth ofthe net and holding the mouth in an open condition. The two part frameuses hole and post members on the respective parts that snap togetherfor easy assembly. In another alternative embodiment, a one partrectangular frame is provided to which four plates having post membersclamp into hole members on the frame. These embodiments have limitedprojections, thereby avoiding the collection thereon of debris withstructure that can easily be loosened by gloved hands.

In accordance with certain principles of the invention, the traps areprovided with net assemblies having a two-stage filter mesh. The netsfor such traps are constructed of an inner net and an outer net. Theinner net provides a first layer of mesh having larger aperture meshopenings so that the inner net captures only the larger items of debris,allowing the smaller items to pass through to the outer net or secondlayer of mesh. The outer net has smaller openings that trap smalleritems of debris that pass through the openings of the inner net. Theopenings in the inner net may, for example, be at least two or threetimes the dimension of the openings in the outer net, or have an areafrom about four to ten times the area of the openings in the outer net.The outer net may also have a greater volume than the inner net, forexample, at least about one fourth larger than that of the inner net.The two stage filter produces a larger effective filtering capacity, inthat the trap does not blind as quickly, holds more material anddistributes the hydraulic loads between the two layers resulting ingreater overall strength. Further, were the first or inner net to fail,the second or outer net retains the ability to trap additional debris.

These and other objectives and advantages of the present invention willbe more readily apparent from the following detailed description of thepreferred embodiments of the invention, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the common features of a debristrapping system of the prior art for the removal of trash or floatablesfrom flowing water.

FIG. 1A is an underground in-line version of the prior art system ofFIG. 1.

FIG. 1B is a floating version of the prior art system of FIG. 1.

FIG. 1C is an end-of-pipe version of the prior art system of FIG. 1.

FIG. 2 is a perspective view of the net assembly of a trap according tocertain principles of the invention.

FIG. 2A is a cross-sectional view along line 2A—2A of FIG. 2.

FIG. 2B is a cross-sectional view along line 2B—2B of FIG. 2.

FIG. 3 is a perspective view of the net of a trap utilizing a net frameconstruction alternative to that of FIG. 2.

FIG. 3A is a cross-sectional view along line 3A—3A of FIG. 3.

FIG. 3B is a cross-sectional view along line 3B—3B of FIG. 3.

FIG. 3C is a cross-sectional of an alternative to FIG. 3B.

FIGS. 4A-4B are cross-sectional views illustrating double netconstruction according to certain embodiments of the present invention.

FIG. 5 is a perspective view of the net assembly of a trap according toan alternative embodiment of the invention.

FIG. 5A is a cross-sectional view along line 5A—5A of FIG. 5.

FIG. 5B is a cross-sectional view along line 5B—5B of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the basic components of one system 40 of the priorart described in the background of the invention above. The system 10includes one or more traps 12, illustrated as two in number, separatelydesignated as traps 12 a and 12 b. The traps 12 a, 12 b are locatedwithin a flow constraining housing or enclosure 11 between inlet 13 andoutlet 14 thereof. The inlet 13 and the outlet 14 are each respectivelyconnected in a known manner to conduits 15 and 16, which may be stormdrain or combined sewer conduits or other structures or the terrain ofthe site. The traps 12 a, 12 b each include a netting assembly 19 formedof a bag-shaped mesh net 17 that is attached to a lifting basket 18.Each of the netting assemblies 19 captures and holds floatable velocityborne debris 20 entering enclosure 11 through inlet 13. The arrows 25indicate the direction of water flow.

Perforations or openings in nets 17 may vary in size depending on theintended use, with sizes generally in the range of from about 0.1″ toabout 2″. Nets 17 are open on the upstream facing end 17 a thereof,toward inlet 13 of enclosure 11. Upper support members (not shown inFIG. 1) are attached to lifting baskets 18 for allowing the nettingassemblies 19 of traps 12 a, 12 b to be lifted out of enclosure 11 forperiodic removal of captured debris. The netting assemblies 19 areconfigured such that the nets 17 provide a large filter area for thesize of the mouth, thereby minimizing head loss. For example, 80 squarefeet of net 17 may be provided for a netting assembly mouth area of 6 ½square feet, resulting in a pressure drop across a net 17 of three orfour pounds.

A bypass weir (not shown in FIG. 1) or screen is typically locatedupstream of traps 12 and on one side of inlet 13 to permit continuedflow in the event that the nets 17 of traps 12 a, 12 b are filled tocapacity with debris. To signal that nets 17 of the netting assemblies19 of traps 12 a, 12 b are in need of replacement or emptying, sensingand signaling mechanisms may be provided. The multiple trap system 10can be configured to provide continuous and uninterrupted capture ofdebris through second trap 12 b after the netting assembly of first trap12 a has been filled and during the process of removing and replacingit. While servicing is being performed, movable panels can be positionedin front of each respective trap 12 a or 12 b being serviced, asnecessary, prior to its removal from enclosure 11. In this way, thesystem 10 is protected against passage of floatable debris during netremoval and replacement.

FIGS. 1A-1C illustrate the basic system 10 of the prior art in threeenvironments. These arrangements are generally described in apublication of the United States Environmental Protection Agency, Officeof Water, No. EPA 832-F-99-037, September, 1999, hereby expresslyincorporated by reference herein.

In particular, in FIG. 1A, an in-line system 10 a is illustrated inwhich the two traps 12 a, 12 b are contained in an enclosure in the formof an underground or subterranean vault 11 a. The vault 11 a includesits inlet 13 a and its outlet 14 a respectively connected to conduits inthe form of buried pipes 15 a, 16 a, for example, of a storm drain. Thein-line traps 12 a, 12 b each include a netting assembly 19 with a meshnet 17 installed in and held in place by a respective lifting basket 18.A lifting bridle (not shown) is attached to upper support members 21 ofthe lifting basket 18 for allowing the netting assemblies 19 of traps 12a and 12 b to be lifted out of vault 11 a through doors 22 a forperiodic removal of captured debris. A bypass screen 23 a is locatedabove the traps 12 a, 12 b to allow flow to divert from the inlet 13 ato permit continued flow in the event that nets 17 of the traps 12 a, 12b are both filled to capacity with debris.

In FIG. 1B, a floating system 10 b is illustrated that is configured tofloat in a body of water in front of a stream, pipe or other watersource from which enters into the body of water a flow of watercontaining trash or floatables to be removed by the system. Thedirection of water flow into and through the system 10 b is alsoindicated by arrows 19. The floating system 10 b also includes two traps12 a, 12 b, shown in a floating hull 11 b that is provided with closedcell foam panels 23 and pontoons to float the hull at the surface 28 ofthe body of water. The traps 12 a, 12 b also each include a mesh net 17held in place within a lifting support 18 a. Because the system 10 b isfloating and the traps 12 a, 12 b are immersed in water, a lessextensive support frame 18 a is substituted for the lifting basket 18 ofsystem 10 a, described above.

In the system 10 b, the hull 11 b has its inlet 13 b extending above andbelow the surface 28 of the water so that trash or floatables at andimmediately below the surface enter through it into the interior of thehull 11 b. The hull 11 b has its outlet 14 b below the water surface 28on the back of the hull 11 b. The inlet conduit 15 is formed of a set ofcurtains 15 b which hang from below the inlet 13 b and from floats 24extending respectively between the hull 11 b on both sides of the inlet13 b to the shore on the opposite sides of the flowing source, connectedto buried concrete conduits (not shown) of a storm drain, for example.The curtains 15 b may extend from the water surface 28 to the bottom 29of the water body and channel water from the source into the inlet 13 b.The traps 12 a, 12 b are supported in the hull 11 b in a manner similarto the way they are supported in the vault 11 a described above. Theycan be lifted out of hull 11 b through grate doors 22 b for periodicremoval of captured debris from the nets 17 thereof.

In FIG. 1C, an end-of-pipe system 10 c is illustrated in which the twotraps 12 a, 12 b are shown in an enclosure in the form of a surfacemounted three-sided concrete headwall and knee wall enclosed cavity 11 chaving an open end that defines its outlet 14 c. The cavity 11 c has itsinlet 13 c connected to a pipe 15 c draining into the cavity 11 c. Thetraps 12 a, 12 b each include a net assembly 19 having a mesh net 17. Afiberglass drain grating 16 c is provided beneath the netting assemblies19 to allow flow to exit each net 17 through its bottom to the outlet 14c of the enclosure 11 c. The net 17 of each netting assembly is attachedto a lifting structure (not shown), which may be similar to the liftingbasket 18 described in FIG. 1A above, or in the form of lifting frame 18a described in FIG. 1B above where the traps 12 a, 12 b are submerged.Door grates 22 c are provided above the traps 12 a, 12 b to permit themto be raised for periodic removal of captured debris. A bypass weir 23 cmay be located above the traps 12 a, 12 b to allow flow to divert fromthe inlet 13 to permit continued flow in the event that traps 12 a, 12 bare both filled to capacity with debris.

In FIGS. 2, 2A and 2B are illustrated netting assemblies for the traps12 for use in systems 10 of the various types illustrated in FIGS. 1A-1Cdescribed above. According to certain aspects of the invention, thenetting assemblies 19 are constructed with a mesh net 17 connected to aframe assembly 30. The frame assembly 30 includes a rectangular framebody having a pair of horizontal top and bottom members 31 and 32,respectively, and a pair of side members 33. The top member 31 is widerthan the bottom member 32, and the side members 33 are tapered from thewider top member toward the narrower bottom member 32, as illustrated inFIG. 2B, for easy installation and removal from the lifting basket 18 orsupport frame 18 a. The side members 33 are also inwardly tapered in thedownstream direction, as illustrated in FIG. 2A, to lock into thesupporting rails as the flow goes through the nets 17. Flow direction isindicated by the arrows 25.

Each of the members 31-33 has a rim 34 on the upstream side thereof anda recessed step 35 on the downstream side thereof. A pattern of holes 36is formed in the steps 35 of each of the members 31-33. Each of themembers 31-33 has associated therewith a plate 37 having a plurality ofprojections in the form of posts 38 arranged in a pattern thatcorresponds to the pattern of the holes 36 in the respectivelyassociated member 31-33 of the frame 30 so that the plates 37 can beconnected to the members 31-33 by snap fitting the posts 38 into theholes 36. The plates 37 are so connected with the edge of the mouth ofthe net 17 between the plate 37 and the respective member 31-33 and theposts 38 extending through holes in the mesh of the net 17, therebylocking the mouth of the net 17 to the frame 30. When so connected, theplates 37 set into the steps 35 so that the tops thereof are flush withthe lip 34 of the members 31-33. When the net 17 is attached to theframe 30, the net extends around the outside of the members 31-33 withthe mouth of the net wrapping around the upstream side of the frame 30to the inside of the frame 30 and between the plates 37 and the members31-33.

The frame 30 may be made of wood and the plates 37 made of metal, butother materials may be used. In one preferred embodiment, the frame 30is formed of an integral piece of molded plastic material. The plates 37may also be formed of molded plastic. The frame 30 securely attaches tothe nets 17 by being formed of elements that clamp together with themesh material of the nets 17 between them, with one of the elementshaving posts or projections thereon against which the other member bearsso that the projections serve as hooks that trap the net between theelements while the other element prevents the net from slipping off theprojections.

An alternative frame structure 18 is illustrated in FIGS. 3, 3A, 3B and3C, in which mesh net 17 is shown connected to a frame assembly 40. Theframe assembly 40 is a two part rectangular frame that includes an innerframe portion 40 a having an array of holes 46 on the upstream facingside thereof and an outer frame portion 40 b having a matching array ofposts on the downstream facing side thereof. The two portions 40 a,40 bof the frame snap together and clamp the mouth of the net 17therebetween. The two parts of the frame 40 are preferably formed of anintegral piece of molded plastic, but other materials may be used.

The frame 40 has a pair of horizontal top and bottom members 41 and 42and a pair of side members 43. The side members are tapered inwardly inthe downstream direction and fit in correspondingly tapered verticalchannels 44 in vertical rails 45 that are part of the lifting basket 18or support frame 18 a. Further, the top member 41 is thicker in the flowdirection (that is, upstream to downstream) than is the bottom member42; and the side members 43 are correspondingly tapered in the downwarddirection to fit into the channels 44, which are similarly tapered, asillustrated in FIG. 3A. As a result of the tapers, the frame 40 of thenetting assemblies 19 fit firmly in the channels 44 of the rails 45 whenin position, but can be loosened by impact and removed with a minimum ofsliding friction. FIG. 3B shows the net 17 wrapped around the outside ofthe frame 40 with the mouth of the net 17 wrapping around the front ofthe frame 40 and extending between the portions 40 a, 40 b thereof fromthe inside. Alternatively, FIG. 3C shows the net 17 wrapped around theinside of the frame 40 with the mouth of the net 17 wrapping around thefront of the frame 40 and extending between the portions 40 a, 40 bthereof from the outside.

As a result of the tapers described above, the greater the forces on thetraps, the more tightly the mesh nets 17 are gripped and the less likelyare the nets to pull out or tear around the posts.

FIG. 4A illustrates a two layered net 17 that includes an inner net 17 aof a course mesh having holes mounted to frame structure 18 c so as toextend through the inside of the frame and with an outer net 17 b of afine mesh mounted to frame structure 18 c so as to extend around theoutside of the frame and thereby enclosing the inner net. The holes inthe inner net 17 a may, for example, be about 1-2 inches in size withthe holes in the outer net 17 b being of about ½ inches in size. Theholes of the inner net 17 a should be at least two to three times largeron a side than those of the outer net, with a cross sectional area of atleast about four times the area of the holes of the outer net. As aresult, large pieces of debris 48 such as plastic bottles, cans, plasticbags, styrofoam cups, etc. only are trapped by the inner net 17 a whilesmaller pieces of debris 49 pass through the larger holes of the innernet 17 a and are trapped by the outer net 17 b.

FIGS. 5, 5A and 5B illustrate netting assemblies for the traps 12 thatare alternative embodiments of the assemblies of FIGS. 2-2B and FIGS.3-3C described above. In FIGS. 5-5B, the traps 12 are each constructedwith mesh net 17 connected to a frame assembly 50. The frame assembly 50includes a rectangular frame body. As with the embodiments above, theframe 50 is preferably formed of an integral piece of molded plastic,but other materials are suitable. The body of frame 50 has a pair ofhorizontal top and bottom members 51 and 52, respectively, and a pair ofside members 53, with the top member 51 wider than the bottom member 52and the side members 53 tapered from top to bottom as was illustrated inthe embodiment of FIG. 2B. The side members 53 are also inwardly taperedin the downstream direction, as illustrated in FIG. 5A. Each of themembers 51-53 has an outside surface 54 having a groove 55 extendingaround the frame 50. On the bottom surface of the groove 55 ispreferably a plurality of projections or posts 56 to help grasp thenetting material, particularly where the frame is formed of plastic orother low friction material. A clamping element in the form of a tensionband 57 lies in the groove 55 in contact with the tips of theprojections 56. The tension band may be of a natural fiber, metal orplastic. Plastic is particularly suitable for the band 57. The net 17extends between the band 57 and the frame members 51-53, so that themouth of the net 17 is locked to the frame 50. When the frame 50 isinserted into the rails of the system, the tapered frame is forcedagainst the frame by the forces produced by the flowing water on the net17 to further clamp the net 17 between the frame 50 and the rail.

Other applications of the invention can be made. Those skilled in theart will appreciate that the applications of the present inventionherein are varied, and that the invention is described in preferredembodiments. Accordingly, additions and modifications can be madewithout departing from the principles of the invention.

Accordingly, the following is claimed:
 1. A disposable netting assembly for a system for trapping floatable debris comprising: a flexible, stretchable, bag-shaped, knitted mesh net having a mouth at one end surrounded by a rim; a frame having an upstream side and a downstream side and comprised of members at a first side thereof coupled to members at an opposing side thereof by members tapering therebetween to facilitate secure retaining of the net to the frame while facilitating removability of the netting assembly from the system; the net being secured around its mouth to the frame; and the net being formed of an inner layer which faces the upstream side of the frame and an outer layer which faces the downstream side of the frame, each layer being formed of a mesh having openings therein, the openings of the inner layer being substantially larger than the openings of the outer layer; whereby fluid carrying debris enters the upstream side of the frame and passes first through the inner layer of the net which removes the substantially larger pieces of debris therefrom, then passes through the outer layer of the net which removes smaller pieces of debris therefrom.
 2. The assembly of claim 1 wherein: the frame has an inside and an outside, and is formed of four members including a top member, a bottom member and two side members, the outside of the side members being tapered inwardly front-to-back and the upstream-downstream sides converging from top to bottom.
 3. The assembly of claim 1 wherein: the net is formed of a high strength and high stretch yarn, with rolled sewn seams and having reinforcing on the seams and on high stress areas of the net.
 4. The assembly of claim 1 wherein: the frame has an inside and an outside, and is formed of a plastic material of four members including a top member, a bottom member and two side members, the outside of the side members being tapered inwardly front-to-back and the upstream-downstream sides converging from top to bottom, the frame having at least one clamping element and a plurality of projections extending either from the frame or the clamping element, the net being secured to the frame with the rim thereof extending over the projections between the clamping element and the frame and around the upstream side of the frame.
 5. The assembly of claim 1 wherein: the frame has an inside and an outside, and is formed of a plastic material of tour members including a top member, a bottom member and two side members, the outside of the side members being tapered inwardly front-to-back and the upstream-downstream sides converging from top to bottom.
 6. The assembly of claim 1 wherein: the openings in the inner layer have an area that is at least four times as large as the openings of the outer layer.
 7. The assembly of claim 1 wherein: the openings in the inner layer have an area that is at least ten times as large as the area of the openings of the outer layer.
 8. The assembly of claim 1 wherein: the outer layer of the net has a volume that is substantially larger than the volume of the inner layer.
 9. The assembly of claim 1 wherein: the outer layer of the net has a volume that is at least approximately twenty-five percent larger than the volume of the inner layer.
 10. The assembly of claim 1 wherein: the rim of the outer layer of the net is attached to the frame so as to be spaced outwardly from the rim of the inner layer.
 11. The apparatus of claim 1 wherein: the frame has an inside and an outside and is formed of a plastic material of four straight members including a top member, a bottom member and two side members, each member having a plurality of holes therein, the net being secured around its rim to the frame, with the mouth thereof extending from the downstream side thereof, around the outside of the frame, around the upstream side of the frame and over the plurality of holes; and the frame having at least one clamping element thereof having a plurality of posts projecting therefrom, said clamping element being secured to the members with the posts snapped into the holes and clamping the rim of the net to the frame.
 12. A disposable netting assembly for a system for trapping floatable debris comprising: a flexible, stretchable, bag-shaped, knitted mesh net having a mouth at one end surrounded by a rim; a frame having an upstream side and a downstream side, an inside and an outside, and being formed of four members including a top member, a bottom member and two side members; the net being secured around its mouth to the frame with the rim thereof extending from the downstream side of the frame, around the upstream side of the frame and over at least portions of the inside and the outside of the frame on each of the four members thereof, and the net being formed of an inner layer which faces the upstream side of the frame and an outer layer which faces the downstream side of the frame, each layer being formed of a mesh having openings therein, the openings of the inner layer being substantially larger than the openings of the outer layer; whereby fluid carrying debris enters the upstream side of the frame and passes first through the inner layer of the net which removes the substantially larger pieces of debris therefrom, then passes through the outer layer of the net which removes smaller pieces of debris therefrom.
 13. A floatable debris trapping apparatus comprising the assembly of claim 12 wherein: the frame has at least one clamping element and a plurality of projections extending either from the frame or the clamping element, the net being secured to the frame with the rim thereof extending over the projections between the clamping element and the frame and around the upstream side of the frame.
 14. The apparatus of claim 13 wherein: the frame is formed of a molded plastic material.
 15. The apparatus of claim 13 wherein: the frame having an inside and an outside, and being formed of a plastic material of four members including a top member, a bottom member and two side members, the outside of the side members being tapered inwardly front-to-back and the upstream-downstream sides converging from top to bottom, the frame having at least one clamping element and a plurality of projections extending either from the frame or the clamping element, the net being secured to the frame with the rim thereof extending over the projections between the clamping element and the frame and around the upstream side of the frame.
 16. The apparatus of claim 13 wherein: the frame is formed of a plastic material of four straight members including a top member, a bottom member and two side members, each member having a plurality of holes therein; the net is secured around its rim to the frame, with the mouth thereof extending from the downstream side thereof, around the outside of the frame, around the upstream side of the frame and over the plurality of holes; and the frame has at least one clamping element thereof having a plurality of posts projecting therefrom, said clamping element being secured to the members with the posts snapped into the holes and clamping the mouth of the net to the frame.
 17. The apparatus of claim 13 wherein: the side members of the frame are inwardly tapered in the downstream direction and wider, in the upstream-to-downstream direction, at the top than at the bottom.
 18. The apparatus of claim 13 wherein: the at least one clamping element includes a set of four straight plates, each having posts projecting therefrom; the holes are on the inside of the members; and the plates clamp the mouth of the net to the frame along the inside of the members.
 19. The apparatus of claim 13 wherein: the at least one clamping element includes a rectangular frame clamping portion having the posts thereon; the holes in the frame and the posts in the frame clamping portion facing in one of the upstream or downstream directions and clamps the mouth of the net therebetween.
 20. The apparatus of claim 13 wherein: the projections extend outwardly from the outside of the frame and the clamping element is a tension band extending around the frame with the mouth of the net and the projections between the band and the frame.
 21. The apparatus of claim 13 wherein: the projections extend outwardly from a groove around the outside of the frame and the clamping element is a tension band extending around the groove with the mouth of the net and the projections between the band and the frame.
 22. The apparatus of claim 13 wherein: the net is formed of a high strength and high stretch yarn, with rolled sewn seams.
 23. The assembly of claim 12 wherein: the frame is formed of an integral piece of molded plastic material.
 24. The assembly of claim 12 wherein: the net is formed of a nylon material.
 25. The assembly of claim 12 wherein: the net is formed of a high strength and high stretch yarn, with rolled sewn seams and having reinforcing on the seams and on high stress areas of the net.
 26. The assembly of claim 12 wherein: the net has reinforcing on the seams and on high stress areas of thereof.
 27. The assembly of claim 12 wherein: the yarn is sufficiently elastic, either due to composition or the way in which it is knitted, to allow the net to deform when clogged with debris and thereby expand to allow flow paths around the trapped debris, thereby reducing pressure drop across the net.
 28. The assembly of claim 12 wherein: the net is formed of a knotless knit pattern of nylon material selected for its minimal effect on the breaking strength of the material.
 29. The apparatus of claim 12 wherein: the openings of the inner layer are at least 1 inch large and at least two times larger than the openings of the outer layer, the openings in the outer layer being at least 0.1 inches large. 